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Aug. 20, 2014 -- Scientists have long studied the link between our genes and our health. Now, in a growing area of scientific research, they're studying the link between the bacteria in our intestines and virtually every disease that ails us.

Bacteria -- along with viruses and fungi -- are microbes, and we're filled with them. For each one of your human cells -- that is, for every cell that's "you" -- there are an estimated 10 microbial cells. They live everywhere in your body: on your skin and inside your mouth, your nose, your genitalia, urinary tract, and intestines.

Together, they form your body's unique collection of microbes, called microbiome -- partly inherited from your mother at birth and partly determined by your lifestyle. Due to their sheer number, there's little question as to whether microbes have an impact on our health. Until recently, though, scientists didn't know much more than that.

Bacteria are most likely the most abundant microbes in your intestines, and they're the focus of most scientific study. Looking at the DNA of the bacteria in stool samples, researchers want to know whether the bacteria cause particular diseases and what we can do to change it.

"There's a good chance your microbiome is associated with every disease you can think of -- diabetes, cancer, autism," says Michael Snyder, PhD. He's the director of Stanford University's Center for Genomics and Personalized Medicine. "And the area where bacteria have a huge impact is your gut."

What Is Gut Bacteria?

Bacteria line your intestines and help you digest food. During digestion, they make vitamins that are vital for life, send signals to the immune system, and make small molecules that can help your brain work.

"Without gut bacteria, we wouldn't be anything. They are a critical part of us and essential to our health," Snyder says.

Ongoing research reveals that people with certain diseases often have a very different mix of bacteria in their intestines compared to healthier people. Researchers are working to define the makeup of gut bacteria in a healthy person vs. the gut bacteria that can point to higher risk or presence of certain diseases.

Some evidence suggests it's not the presence or absence of one particular type of bacteria that makes a microbiome a healthy one, but rather the diversity of bacteria.

"If you have a wide array of bacteria that can break down lots of different food sources, produce lots of different molecules that help mature your immune system, and produce the molecules that your brain needs to function properly, you can see how that would potentially be a benefit over a less diverse gut microbiome," says Joseph Petrosino, PhD. He's the director of the Alkek Center for Metagenomics and Microbiome Research at Baylor College of Medicine.

Microbiome and Disease

Microbiome studies are still too new to reveal whether certain bacteria might cause disease or whether disease might breed certain bacteria -- or whether the relationship is something else altogether. For now, scientists are only making links between a person's bacterial makeup and the presence of certain diseases. Regardless of whether a cause-and-effect relationship is found, looking at gut bacteria could become a way for doctors to diagnose certain diseases earlier and more accurately, Petrosino says.

Research has shown links to colon cancer, Crohn's disease, ulcerative colitis, diabetes, and obesity. While these conditions might seem more related to the intestines or metabolism, gut bacteria has been linked to diseases throughout the body.

"Many diseases -- of the skin, lungs, joints, and other tissue -- are caused by inflammation," Petrosino says. "A bacterial imbalance can lead to elevated inflammation that can advance disease."

A recent study shows that people with untreated rheumatoid arthritis, an inflammatory autoimmune disease, have more of a particular inflammatory bacteria in their intestines and less of a known beneficial bacteria than their healthy counterparts.

Researchers have also uncovered connections between intestinal bacteria and anxiety, depression, obsessive-compulsive disorder, ADD, autism, and Alzheimer's disease, among others. Some chalk up this link to intestinal bacteria's ability to make small molecules (called metabolites) that can reach the brain and impact how it works.

Care and Feeding of Your Microbiome

As the roster of diseases linked to intestinal microbes continues to grow, the burning question is this: Can you change your gut bacteria and cure or get rid of your risk for a particular disease?

"If you make a long-term dietary change -- for example from a high-fat, high-sugar diet to a leaner, more high-fiber diet -- it's possible that you could reshape your microbiome, giving it a healthier profile," Petrosino says. This could improve immune function, lower inflammation, and lead to overall better health. Not just a healthy diet, he says, but a more varied diet may be key to fostering a diverse and healthy microbiome. Exercise might diversify gut bacteria, too, says a recent study that showed athletes had more varied intestinal microbes than their non-athlete peers.

Understanding the microbiome won't just highlight the importance of diet and exercise. It could lead to advances in medical treatments, too. For example, doctors now do fecal transplants on people with difficult-to-treat c. difficilebacterial infections. The doctor puts a solution of healthy feces into the sick person's colon through their rectum. The feces have healthy gut bacteria that can fight the infection. Ongoing research is looking at using this procedure in other conditions.

Some scientists believe analysis of your microbiome will one day be as common as routine blood tests. Doctors could discuss the results with you and the long-term management of your gut bacteria. Current research could one day lead to customized probiotics that would offset whatever disease-promoting microbiome you might harbor.

"You could envision a therapy, where people are actually taking specific microbiota, that actually helps them prevent obesity or diabetes," Snyder says.